Selection of Splits

At the sequencing stage, it is necessary to realize selection of splits in each column to decrease the tree of sequences before calculation of the columns for estimation of expenditures on separation. 

As a result of such selection, we obtain several preferable sequences that at the completing stage of synthesis have to be compared with each other by the expenditures in order to choose the optimum one. 

Several heuristic rules directed to decrease energy and capital expenditures are used for selection The first rule is for the mixtures having the region of two liquid phases it is necessary to use the most interesting splits at heteroazeotropic and heteroextractive distillation described in the section 8.4.5. Such splits separate, in the cheapest way, the mixture into components. The second rule is to exclude sphts for which one of the products is binary azeotropic mixture, if other splits 

The rest of heuristic rules are less obvious. The fourth rule sequences with minimum number of columns are preferable. Hence, it follows that splits with distributed components have to be excluded if there are ones without them. Capital costs at minimum number of columns are the smallest. However, energy expenditures in this case are not always the smallest. For zeotropic mixtures, the situation is the same as for azeotropic flowsheets with prefractionator are more profitable sometimes than without it (see Section 8.2). Therefore, the designer has to decide him- or herself if he or she should use this rule at the stage of automatic sequences. 

The flowsheet (a) has considerably heavier energy expenditures because of the big necessary reflux number in the third column and big secondary recycle flow rates of azeotrop 13 and components 1 and 2. Besides that, this flowsheet is bigger by one column. 

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For homogeneous azeotropic mixtures, the system of automatic sequencing also includes the stage of determination of possible product compositions (presynthesis) and stage of heuristic selection of splits. 

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